2.2 Applications and skills

2.2.1 Identifying and drawing biochemicals

  • There are a number of biochemicals that you need to be able to draw, and many more that you should be able to recognise for your exam.
  • On this page, you can practise the drawing/identification skills associated with each subtopic of molecular biology.

Skill 2.1.1a: Draw molecular diagrams of glucose, ribose, saturated fatty acids and amino acids

Molecule name and diagram

Drawing tips

Figure 2.2.1a – α-glucose, C6H12O6Figure 2.2.1a – α-glucose, C6H12O6

  • You do not need to include carbon numbers in your drawings – they are there to guide you.
  • C-1 is part of the ring, not in the ‘tail’.
  • In the α- form, the hydroxyl (-OH) groups point down, down, up, down from C1 through C4.
  • In the β- form, the hydroxyl groups point up, down, up, down from C1 through C4.

Figure 2.2.1b – Ribose, C5H10O5Figure 2.2.1b – Ribose, C5H10O5

  • See the first two points above.
  • The hydroxyl groups point up on C1 and down on C2 and C3.

Figure 2.2.1c – Saturated fatty acidFigure 2.2.1c – Saturated fatty acid

  • The only double bond is between C and O in the carboxyl group.
  • The carboxyl group is always at the end of the chain.
  • The rest of the chain is made of single bonds of C and H.

Figure 2.2.1d – Generalised amino acidFigure 2.2.1d – Generalised amino acid

  • R represents any of 20 structures. You do not need to know specific amino acid structures.
  • One carbon atom is bonded to the amine group (NH2), the R group, and the carboxyl (COOH).

Skill 2.1.1b: Identify sugars, lipids and amino acids from molecular diagrams

State which biochemical displays the following characteristics:

  • Long chain of hydrophobic C-H bonds with a terminal carboxyl group (COOH).
  • Chains or rings containing C-H bonds, C-OH bonds and one double CO bond.
  • Variable molecular structures containing nitrogen, carbon and oxygen with a terminal amino (NH2) group and a carboxyl (COOH) group.

Figure 2.2.1e – Can you identify which of the diagrams represents a sugar, lipid and amino acid? Click on the images for the answers.

Skill 2.1.3: Compare cellulose, starch and glycogen using visualisation software

The images in Figure 2.2.1f show the three-dimensional structure of three polysaccharides: cellulose, amylose and glycogen.

You can interact with these molecules at:


left middle right

Figure 2.2.1f – Can you identify the polysaccharide from its 3D model? Scroll over the images for the answers.

Skill 2.1.4: Draw molecular diagrams to show the formation of peptide bonds

  • Peptide bonds form between two amino acids in the formation of polypeptides.
  • The amino group of one amino acid joins the carboxyl group of another, releasing a water molecule. This is a condensation reaction.

peptide bond

Figure 2.2.1g – Formation of a peptide bond

  • When you draw a peptide bond, be sure to have the C-N bonded to each other.
  • The orientation of R groups (up or down) is not important.

drawing diagramsFigure 2.2.1h – Drawing diagrams
Always use a pencil when drawing diagrams.

Study tip

Go through your syllabus and make a list of all the diagrams you need to be able to draw for your exam. Draw all the diagrams without labels or annotations. You can use them to test yourself on key concepts later.

Course link

For Topic 2, you also need to know how to draw DNA and individual nucleotides. Refresh your memory in 2.1.6.